The mean preferred add was +0.58 D over the distance Rx. The SD was 0.54 D, and the median was +0.50 D. The range of preferred adds was 0 to +2.00 D. The distribution of preferred adds is shown in Fig. 4. The mean difference between AS and AR at the preferred add was +0.67 D (SD = 0.52 D, median = +0.75 D).
Comparison of Preferred Add with Dynamic Retinoscopy Prescription Guidelines
Comparisons of the outcomes of various guidelines with the preferred add are summarized in Table 1. The mean lag at the 2.50 D stimulus was 0.92 D (SD = 0.43, median = 0.75 D). Thus, the mean add based on the guideline of prescribing an add equal to 0.25 D less than the amount of the lag was 0.68 D. With the mean preferred add being +0.58 D, the mean difference between this guideline and the preferred add was 0.10 D (n = 80, SD = 0.70 D). The distribution of these differences did not show skew or kurtosis, as suggested by the low skewness and kurtosis values seen in Table 1.
The Pearson correlation coefficient for differences between the add recommended by the MEM lag− 0.25 D guideline and the preferred add with the mean of those two values was −0.25 (p < 0.05). A plot of differences as a function of means is shown in Fig. 5. The correlation was significant, although low, because some subjects with low lags preferred adds higher than predicted by the guideline and some subjects with high lags preferred adds lower than predicted by the guideline.
Not all subjects obtained the result of 0.25 D “with” motion because some had no lag with zero add and some still had an AS minus AR difference of >0.25 D with a +2.00 D add. The mean add was +1.14 D (SD = 0.52 D) for the 59 subjects who did have a point at which 0.25 D “with” motion was observed. The mean difference between the add to 0.25 D “with” motion and the preferred add was +0.54 D (SD of differences = 0.83 D). The correlation of differences between guideline add and preferred add with the mean of those two values (Table 2) was not statistically significant, indicating there was no relation of the difference between guideline and preferred add with the magnitude of findings.
Similarly, not all subjects obtained 0.50 D “with” motion. For the 69 subjects who did, the mean add was +0.73 D, with an SD being 0.59. The mean difference between the add to 0.50 D “with” motion and the preferred add was +0.16 D, with an SD of the differences being 0.77 D.
Because there were some subjects who did not show a high correlation of AR and AS, we could ask whether the results would be any different for only those subjects who did. The adds recommended by the three prescription guidelines were compared with the preferred add for only the 68 subjects whose Pearson correlation coefficient of AR vs. AS was >0.80. The mean preferred add was +0.61 D (SD = 0.52, median = +0.50). For the guideline of prescribing 0.25 D less than the amount of the lag, the add averaged 0.01 D less than the preferred add (n = 68, SD of the differences = 0.64, median difference = 0). The add obtained at the point of 0.25 D “with” motion averaged 0.52 D more than the preferred add (n = 55, SD of differences = 0.76, median difference = +0.75). The add at 0.50 D of “with” motion averaged 0.09 D more than the preferred add (n = 59, SD of differences = 0.76, median difference = 0).
Modified Thorington Near Dissociated Phoria
The mean modified Thorington near dissociated phoria through the distance correction was 1.2Δ exo (SD = 3.4, range = 13Δ exo to 5Δ eso). There were 26 subjects with an eso finding at near through zero add and 54 with ortho or exo. Mean dissociated phorias at each AS level are shown in Fig. 2. The mean phoria at the preferred add was 2.1Δ exo (SD = 3.1Δ, median = 2Δ exo). The mean preferred add for the 26 subjects with eso at near through distance correction was +0.77 D (SD = 0.55, median = +0.63 D). For the 54 subjects who had ortho or exo at near through zero add, the mean preferred add was + 0.49 D (SD = 0.51, median = +0.25 D).
There were 28 subjects who had a phoria of 1Δ exo within the range of adds presented, including seven subjects who had a phoria of 1Δ exo with zero add. For those 28 subjects, the minimum add to 1Δ exo averaged 0.07 D less than the preferred add (SD of differences = 0.75, median difference = 0).
There were 38 subjects who had a phoria of 2Δ exo within the range of adds presented. Among these 38 subjects, the phorias with zero add ranged from 2Δ exo to 4Δ eso. The minimum add to 2Δ exo averaged 0.12 D less than the preferred add (SD of differences = 0.68, median difference = −0.25 D).
There were 32 subjects who had a phoria of 3Δ exo within the range of lenses presented. Among these 32 subjects, the range of phorias through zero add was from 3Δ exo to 4Δ eso. The minimum add to 3Δ exo averaged 0.28 D more plus than the preferred add (SD of differences = 0.70, median difference = +0.25 D).
Comparison of the add recommended by phoria results with the preferred add was also performed for a subset of subjects with eso at near through zero add, as prescription of plus adds is more likely in eso at near. The results are shown in Table 3. In general, the adds to low amounts of exo were a little more plus than the preferred add, but the numbers of subjects were low.
Comparison of Preferred Add with Modified Pratt Analysis Add
The add recommended by the modified Pratt analysis using dynamic retinoscopy and dissociated phoria averaged 0.04 D less plus than the preferred add (SD of differences = 0.78 D, median difference = 0).
Comparison of Results with Studies on MEM
MEM dynamic retinoscopy is usually performed with patients viewing through their distance refractive corrections, so MEM results from previous studies could be compared with the zero add (2.5 D AS) condition in the present study, for which the mean was 0.92 D. Average MEM lag values in the literature are 0.23 and 0.34 D in two studies with children29,30; 0.50, 0.56, 0.74, and 0.89 D in four studies in young adults31 – 34; and 0.35 D in a study of patients with a wide range of ages.35 Thus, the average MEM finding with zero add in the present study is at the high end of the range of values reported for young adults.
Use of MEM-LN Dynamic Retinoscopy for AR/AS Functions
Laboratory studies on AR as a function of AS have typically shown a lag of accommodation for accommodative stimuli of about 1 D or more, and slopes <1 D/D.36 – 43 Laboratory studies have differed from the present study in that (1) they have often varied AS by changing distance or using a Badal system, (2) they have used a wider dioptric range of accommodative stimuli than is found in the present study with a 40 cm test distance and typical near-point adds, and (3) they have sometimes used monocular rather than binocular viewing. AR varies with many optical and non-optical factors, such as blur, target size, convergence, perception of proximity, voluntary effort, and depth of focus,43 so one would expect AR/AS functions to vary with different measurement methods.
Haynes10,11 presented seven representative cases with the MEM-LN dynamic retinoscopy procedure, but he did not report population summary data such as mean slopes of AR as a function of AS. In his seven representative cases, the slopes ranged from 0 to 0.55 D/D. The present study found a mean slope of 0.51 D/D (SD = 0.18), with a range of slopes of 0.13 to 0.90 D/D. A preliminary study of MEM-LN dynamic retinoscopy including 20 young adult subjects34 found a mean slope of 0.49 D/D (SD = 0.17), and a range of 0 to 0.68 D/D. These means could also be compared with a study using an open-view autorefractor with binocular fixation, a 40 cm test distance, and adds in 0.25 D steps from 0 to 2.50 D.44 In the autorefractor study, the mean AR/AS slope was 0.51 D/D (SD = 0.16), and the range of the slopes was 0.11 to 0.80 D/D.
Comparison of Dynamic Retinoscopy Prescription Guideline Results with Preferred Add
This study compared values from common guidelines for prescribing plus adds for non-presbyopes with the subjectively preferred plus add power. The guideline of subtracting 0.25 D from the lag with distance correction averaged very close to the preferred add, but there were a number of subjects for whom that guideline differed significantly from the preferred add, as shown by the high SD of differences from the preferred add. Subtracting 0.25 D from the lag gave results slightly closer to the preferred add than finding the add that yielded 0.50 D “with” motion. Both of those guidelines were better than using the add to 0.25 D “with” motion. However, with all three methods, there was a high SD of the differences from the preferred add. The results of this study suggest that subtracting 0.25 D from the lag or adding plus to 0.50 D of “with” motion appears to give reasonable starting points for near-point plus in most persons, but follow-up testing with a trial frame may be necessary to confirm findings with individual patients in the clinical setting.
Comparison of Preferred Add with Adds Derived from Dissociated Phoria Findings
The results with dissociated phoria testing indicated, on average, a slightly better agreement of the add to 1 or 2Δ exo with the preferred add than of the add to 3Δ exo, using the modified Thorington testing method. Here again, the SDs of the differences from the preferred add were relatively high so that the add to 1 or 2Δ exo may be useful as a starting point, but confirmation with subjective evaluation in a trial frame would be indicated for individual patients. The fact that the mean phoria with the preferred add was 2.1Δ exo and the median was 2Δ exo appears to provide further support for low exo being a desirable phoria level. However, because the number of subjects with esophoria at near through zero add was fairly low, it is possible that the overall phoria guideline results may not generalize well to eso cases where prescription of plus adds is more likely.
Pratt System of Accommodation and Convergence Analysis
A theoretical advantage of the Pratt analysis system is that it uses both accommodation and vergence findings. The mean difference of nearly zero between the preferred add and the modified Pratt analysis add suggests that it too, on average, is a reasonable starting point for near-point plus adds, but the high SD of the differences suggests that in many cases, additional testing is needed to arrive at the best lens power to be prescribed.
Possible Applications to Myopia Control
Studies on the use of bifocals and progressive addition lenses have shown that rates of myopia progression are reduced by plus adds in children with esophoria at near.4,45 Prospective studies showing that outcome have used one add power for all experimental group subjects.46 – 53 However, practicing optometrists attempting myopia control with bifocals have based the add power prescription on the individual characteristics of the patient. Prescription criteria for add power used by practicing optometrists in myopia have included standard optometric tests such as dissociated phoria, binocular cross cylinder, negative relative accommodation, and positive relative accommodation.13,54
Some investigators are developing models of accommodation and vergence relationships and parameters to predict the best adds for slowing myopia progression in different individuals based on their accommodation and vergence characteristics.55 – 57 It has been suggested that the greatest reduction in childhood myopia progression with multifocal lenses would most likely be achieved with add powers that are individually prescribed for optimal near-point visual function and comfort.13,58 Perhaps, guidelines derived from dynamic retinoscopy and/or dissociated phoria findings could be adapted to develop procedures that would recommend individual myopia control adds.
Effects of Plus Adds in Non-presbyopes
There were debates on the effects of low plus adds on near-point visual performance in the 1970s.59 – 65 Today, the usual approach is to use plus adds in cases where esophoria at near and/or high lag of accommodation are found.8 Of particular interest relative to the present study are three studies that reported that plus adds based on dynamic retinoscopy resulted in improvements in various measures.
Sohrab-Jam66 studied eye movement patterns during reading in 38 4th- and 5th-grade boys who were behind grade level in reading achievement. The 19 boys who had higher lags of accommodation according to the dynamic retinoscopy procedure known as book retinoscopy showed improvements in numbers of fixations and regressions and in reading rate with +0.50 D lenses compared with plano lenses. The 19 subjects with low lags of accommodation or leads of accommodation did not show such improvements with +0.50 D lenses.
Caden et al.67 studied performance in 5- to 8-year-old children on the Winter Haven Copy Forms Test, in which geometric forms are copied. Each subject was tested first with lenses equal in power to the MEM retinoscopy lag and then again with the habitual prescription. Scores for the completed forms, evaluated based on the manual for the test, were better with the lenses based on dynamic retinoscopy for the group of 15 subjects.
Price and Maples68 used measurements of muscle strength performed by a physical therapist as an assessment of physiological stress during reading in 5th-grade students. Strength of arm muscle contraction was assessed first with subjects looking at distance and then when reading aloud from Gray Oral Reading paragraphs through adds ranging from +0.25 to +1.25 D. Twenty of 33 subjects showed improved muscle testing results with the plus adds. The add power that yielded the best muscle test outcome showed a significant correlation with lag from Nott retinoscopy (r = 0.50, p = 0.02), with the best add tending to be slightly lower than the Nott retinoscopy lag. Although the exact mechanism by which this physiological measure may relate to visual function is unclear, it is interesting that it correlated with dynamic retinoscopy results.
It may also be noted that plus adds have been reported to reduce near-point asthenopic symptoms in accommodative insufficiency. For example, Daum's retrospective record review of accommodative insufficiency showed that 15 of 17 patients treated with plus adds had total or partial relief of symptoms.69 Two prospective studies found plus adds reduced asthenopia levels in subjects with accommodative insufficiency.70,71
Variables That May Have Affected Results
Limitations of the present study include variables that may have affected study results. Because the lateral separation of optical centers of the plus add spectacles were 59 mm in one and 62 mm in the other set of spectacles, small prismatic effects would have been present for subjects with near PDs that differed from 59 or 62 mm. For example, Prentice rule72 suggests that a subject with a near PD of 56 mm would have had a prismatic effect of 0.6Δ base-out with the +2.00 D adds with a 59 mm lateral separation of optical centers.
Subjects read words on a Welch Allyn adult-level dynamic retinoscopy card, which contains 28 words of two to five syllables each. It was hypothesized that words of such length would better maintain the subject's attention than shorter words. However, because the words had to be read more than once, it is possible that some subjects may have had decreased attention later in the testing procedure, which in turn may have affected results.
Subjects were recruited for the study irrespective of presence or absence of near-point visual symptoms, or of diagnosed accommodation or vergence disorders. It was hypothesized that asymptomatic subjects would be likely to prefer no add or a low-power add, thus giving a wide range of preferred adds. Further, the goal of the study was to compare the results of the various guidelines with the preferred add in isolation from other test results or considerations. In light of the high SDs of the differences between guidelines and preferred add, one could speculate that the variability of the differences might have been less in subjects with near-point symptoms.
One could also wonder about the criteria that individual subjects used in picking the preferred add and how repeatable the determination of the preferred add may be. Subjects were instructed to make their selection based on ease and comfort of reading. How that instruction was interpreted may have varied from subject to subject.
Confirmation Procedures for Plus Adds
In the present study, various guidelines for prescribing plus adds for non-presbyopes were compared with the subjectively preferred add. A subjectively preferred add was used in the comparison because many clinicians use subjective preference in a similar way to confirm add powers suggested by clinical test results. Other methods to confirm add power in the clinical setting include improvement on various tests, such as eye movements, stereopsis, or near point of convergence.13,66,73,74 Perhaps, an interesting topic of future study would be comparisons and repeatability of adds recommended by various confirmation procedures, including subjective preference.
This study sought to identify dynamic retinoscopy guidelines that could be used to find the plus add power providing the best near-point visual comfort in non-presbyopes. Because the guideline of adding plus to 0.25 D with motion on dynamic retinoscopy differs by about half diopter, on average, from the subjectively preferred lens, it does not appear to be a useful guideline for prescription of plus adds for non-presbyopes. Subtracting 0.25 D from the MEM lag of accommodation and adding plus to a 0.50 D with motion yield adds that average close to the subjectively preferred add, but the SDs of the difference were relatively high. Either of the latter two guidelines could be a reasonable starting point for the prescription of near-point plus adds for non-presbyopes, but follow-up trial frame testing to confirm or adjust the add power is advisable.
David A. Goss
School of Optometry, Indiana University
800 East Atwater Avenue
Bloomington, IN 47405
This study was supported by a grant from the College of Optometrists in Vision Development.
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Keywords:© 2012 American Academy of Optometry
accommodative response; dynamic retinoscopy; monocular estimation method; near-point lenses; plus lenses